Challenge: Become The Deep State
I may/may not be the first to come up with this idea or challenge. I've never heard of it before, but this was something I am in process of testing.
Step 1: Acquire vast amounts of wealth. Attain funds thru war and attacking caravans with a fast party (using the arrow keys to micro move cut them off from escape in certain regions on map is very useful). Use that money to open businesses. Trade builds work too but has been slower in my experience.
Step 2: Use wealth to open businesses and establish caravans to attain a good positive passive income. Try to open businesses where there will likely be the least war. Disband expensive units for better results. Don't lose battles, play it safe.
Step 3: Use amassed wealth to make peace with the 1 faction you attacked the caravans of.
Step 4: Be Mercenary for a kingdom that's not the largest nor smallest. Use them to build your renown and clan tier. Can't hurt to establish good relationships with members.
Step 5: Join a kingdom. If you don't want to join the kingdom you're currently a mercenary with, end your contract and join a kingdom that is expanding and doing well. At the ruler may give you a castle upon swearing fealty to him/her depending on your renown.
Step 6: Get as good relation as you can with the ruler clan by donating 10k+ gold to each family member. Do their quests, help them in battle when you can. They will be more likely to override the council votes and reward you with more fiefs.
Step 7: once you have amassed about 300k gold, travel among the most power rival kingdom that you're not at war with. Buy all their food and animal stock from all of their cities and villages (except horses/mules/camels). Sell them any non-food items as needed but NO FOOD ITEMS.
Step 8: stockpile and/or sell your food items to your kingdom. This way you're basically taking the food away from your future enemy and have their armies starve. Yes their food will come back, but it's a question of whether it will come back fast enough to avoid starvation in their settlements. It can't hurt to buy all their food whenever you can.
Step 9: At this point, repeat this process with other kingdoms you're likely going to war with. At some point you'll either become a great lord or even elected king if all the families like you enough.
Step 10: Congrats, you're the deep state now. You can punish and undermine other nations via economics and essentially become the medieval version of the Deep State.
The goal is to choose which kingdom you want to be the leader of someday, or at least which one you want to have take over Calradia. You could also stab them in the back by breaking away from them, if you have a good amount of territory and military might.
I tested this, so far I'm between Step 8 and 9. I did buy out all of Vlandia's food and the Western Empire basically walked in and took many territories from them that I never seen them take before.